TRPA1 (formerly known as ANKTM1) (Story, 2003) belongs to the family of the transient receptor potential channels (TRP ion channels). In mammals this family consists of 28 different proteins grouped into six subfamilies by means of amino acid sequence homology: TRPC (canonical, seven members), TRPM (melastatin, eight members), TRPV (vanilloid, six members), TRPA (ankyrin, one member), TRPML (mucolipin, three members), and TRPP (polycystin, three members) (Ramsey, 2006; Clapham, 2007; Wu, 2010). The TRP proteins are composed of six putative transmembrane domains, a pore-forming loop between the fifth and sixth domain, and intracellularly located N- and C-termini (Gaudet, 2008). They apparently all assemble as tetramers to establish ion channels that mediate the flux of cations, especially Na+ and Ca2+, across membranes. Typically, activation of these channels leads to depolarization and initiates a multitude of cellular responses (Clapham, 2003).
TRPA1 is the only mammalian member of the “ankyrin” subfamily. The protein contains a high number of ankyrin repeats (at least 14) in its N-terminus. These are supposed to interact with the cytoskeleton or to modulate ligand binding (Howard and Bechsted, 2004; Sotomayor, 2005, Lishko, 2007). The channel is expressed in subpopulations of dorsal root, trigeminal, and nodose ganglia neurons, especially in C- and Ad-fibers of the pain pathway, were it plays an important role in nociception, neurogenic inflammation, and skin hypersensitivity (Story, 2003; Jordt, 2004, Bautista, 2006). In addition, TRPA1 is expressed in hair cells of the inner ear, endothelial and epithelial cells (Corey, 2004; Atoyan, 2009; Kwan, 2009)
TRPA1 responds to a wide variety of stimuli. For instance, it is activated by a multitude of exogenous and endogenous chemicals. Many of these chemicals are highly reactive electrophiles that form covalent adducts with intracellular cysteine residues of TRPA1 (Hinman, 2006; Macpherson, 2007). They are structurally quite diverse, including cinnamaldehyde, allicin and allyl isothiocyanate (the main pungent ingredients of cinnamon, garlic and mustard oil, respectively), environmental irritants such as acrolein, and endogenous compounds like 4-hydroxynonenal or certain prostaglandins (Bandell, 2004; Jordt, 2004; Bautista, 2005; Macpherson, 2005; Trevisani, 2007; Taylor-Clark, 2008). In its role as a sensor for reactive and therefore potentially harmful chemicals it is conserved from flies to men (Kang, 2010). However, TRPA1 is also activated by some more “classical” (lock and key) ligands, e.g. menthol and p-hydroxybenzoic acid esters, the so-called parabens (Karashima, 2007; Fujita, 2007).
Moreover, TRPA1 is activated downstream of certain G protein-coupled receptors (in a receptor-operated manner), by an increase in intracellular calcium, and through intracellular acidification (Zurborg, 2007; Wang, 2008; Wang, 2010). Further, it is proposed to be activated by noxious cold (<17° C.) and therefore presumably involved in thermosensation. Additional studies point to a role for TRPA1 in the mechanisms of mechanical and cold hypersensitivity produced by skin irritation or inflammation (Bautista, 2006; Petrus, 2007; da Costa, 2010; Wei, 2011).
Just recently a TRPA1-associated channelopathy was reported (Kremeyer, 2010). A gain-of-function mutation in the fourth transmembrane domain leads to a familial episodic pain syndrome. This report further strengthens the relevance of the channel in human pain signalling pathways.
Certain substances of cosmetic and/or pharmaceutical compositions can cause skin irritation if they are applied to the skin, especially the face. This may lead to unpleasant sensations like stinging, burning, and itching, especially in persons with sensitive skin. It is known that these nociceptive sensations are at least to a certain extent mediated by TRPA1. Examples of such substances in cosmetic compositions are emulsifiers, detergents, preservatives, anti-aging compounds, depilation agents, and peeling agents such as α-hydroxy acids. Preservatives such parabens are known triggers of skin irritation (Sone, 1990; Lee, 2007). In particular, Fujita et al. (2007) reported that parabens cause pain sensation through activation of TRPA1. Furthermore, the skin as a barrier of the organism is permanently affected by environmental factors such as ultraviolet (UV) radiation, extreme temperature, and weather conditions or polluting emissions.
Sensitive skin is a complex phenomenon because it is a heterogeneous and self-diagnosed medical condition (often occurring without measurable signs of skin inflammation). Consumers claiming that they have sensitive skin are a growing problem for the cosmetic as well as the pharmaceutical industry. Symptoms like stinging, burning, and itching may lead to dissatisfaction and thereby influence life quality and consumer preferences (Farage and Maibach, 2010). Increasing rates and accumulating reports of sensitive skin give rise to a constant need for new desensitizing (soothing) agents. It is reasonable to assume that up to 50% of people living in the industrial nations possess sensitive skin.
Numerous skin irritation-reducing compounds are established in the technical field referred to, but researchers are constantly looking for alternatives. Known inhibitors of TRPA1 include AP-18 (Petrus, 2007), HC-030031 (McNamara, 2007) and the related compounds A-967079 (WO 2009/089082) and CHEM-5861528 (Wei, 2009). Numerous other TRPA1 antagonists have been described in following exemplary patent applications: WO 2009/089083 (filed by Abbott Laboratories), WO 2010/141805 (Janssen Pharmaceutica), WO 2010/138879 (Hydra Biosciences), and WO 2010/125469 (Glenmark Pharmaceuticals).
Some of the above-mentioned compounds exhibit antagonist activity at TRPA1 at least to some extent, but may be insufficient and unsatisfactory in the retainability of the inhibitory effect. Furthermore, some of the TRPA1 antagonists/inhibitors known in the art may be insufficient with regard to their efficacy, their duration of action, their scent, their taste, their selectivity, their solubility, and/or their volatility. Accordingly, there is a need for TRPA1 antagonists/inhibitors that may overcome one or more of these drawbacks. Furthermore, several studies using TRPA1 inhibitors demonstrated a potential role of TRPA1 in the treatment of pain and analgesia. Hence, there is a strong demand in the art for providing alternative TRPA1 antagonists/inhibitors that can be used as soothing agents.